In recent years, wireless devices using wide-band millimeter waves (30 GHz to 300 GHz) have become widespread. The millimeter-wave radio technology has been expected to be especially applicable to Gigabit-class high-rate radio data communication such as radio transmission of high-resolution images (for example, see Non-patent literatures 1 and 2).
However, the millimeter waves, which have high frequencies, have a high rectilinear propagation property, and thus raising a problem when radio transmission is to be implemented indoors. In addition to having the high rectilinear propagation property, the millimeter-wave signals are significantly attenuated by a human body or other objects. Therefore, when a person stands between the transmitter and the receiver in a room or the like, it is impossible to obtain an unobstructed view, thus making the transmission very difficult (shadowing problem). Since this problem is caused as a result of the higher rectilinear propagation property of radio waves resulting from the use of higher frequencies as well as the change in the propagation environments, the problem is not limited to the millimeter wave band (30 GHz and above). Although it is not easy to clearly specify the transition frequency, it has been said to be around 10 GHz. Meanwhile, according to recommendations of the International Telecommunications Union (“Propagation data and prediction methods for the planning of indoor radio communication systems and radio local area networks in the frequency range 900 MHz to 100 GHz,” ITU-R, P.1238-3, April, 2003), the power loss coefficient, which indicates the attenuation amount of a radio wave with respect to the propagation distance, is 22 for 60 GHz in an office, while it is 28 to 32 for 0.9 to 5.2 GHz. Considering that it is 20 in the case of free-space loss, the effects of scattering, diffraction, and the like are considered to be small for high frequencies in the order of 60 GHz.
To solve the problem described above, for example, Patent literature 1 discloses a system in which more than one transmission path is provided by installing a plurality of receiving units in the receiving device, so that when one of the transmission paths between the transmitting device and the receiving units is blocked, the transmission is performed by another transmission path. Furthermore, as another method for solving the problem, Patent literature 2 discloses an invention to secure plural transmission paths by installing reflectors on walls and a ceiling.
In the method disclosed in Patent literature 1, it is very difficult to continue the communication when the area at and around the transmitting device is shielded or when all of the installed receiving units are shielded. Meanwhile, the method disclosed in Patent literature 2 requires the user to take the trouble to install the reflectors with consideration given to the positions of the transmitter and the receiver and the like.
However, recent studies on the propagation properties of millimeter waves have found out that there is a possibility that reflected waves can be utilized without intentionally installing the reflectors. FIG. 9 is a schematic diagram of a communication system using a millimeter wave band. Each of a transmitter 91 and a receiver 92 has a wide-angle antenna. FIG. 10 shows an example of a delay profile of the system using the wide-angle antennas shown in FIG. 9 when the system is used indoors. In the system using the wide-angle antennas shown in FIG. 9, the received power of the dominant wave, which is arrives faster than any other waves, is larger than that of any other waves as shown in FIG. 10. After that, although delayed waves such as the second and third waves arrive, the received power of these waves is smaller than that of the dominant wave. These second and third waves are reflected waves from the ceiling and the walls. This situation is remarkably different from the propagation environment of radio waves having a lower rectilinear propagation property, such as 2.4 GHz band used in wireless LANs (Local Area Networks). In 2.4 GHz band, it is very difficult to clearly separate waves in their directions of Arrival because of the effects of diffraction and multiple reflections. In contrast to this, in the millimeter waves having a high rectilinear propagation property, although radio waves are relatively clearly distinguished in their directions of Arrival, the number of delayed waves is limited and the received-signal level of the delayed waves is relatively small.
Therefore, in communication systems using a frequency band around or higher than 10 GHz such as millimeter waves, when the direct wave (dominant wave) is shielded, the receiver must point a narrow beam having a high directive gain to the direction of Arrival of a reflected wave to ensure a sufficient received-signal level so that the transmission can be continued by using the reflected wave. However, in order to eliminate the necessity for the user to take the trouble in regard to the relative positions of the transmitter and receiver, and the like, the beam forming technology capable of dynamically controlling the direction of a narrow beam is indispensable.
In the beam forming, it is necessary to construct an antenna array. For millimeter waves having a short wavelength (e.g., 5 mm in the case of frequency of 60 GHz), the antenna array can be implemented in a small area. Phase shifter arrays and oscillator arrays for use in such antenna arrays for millimeter waves have been developed (for example, see Non-patent literatures 3 and 4).